Various methods are known for filling a receptacle with a net weight of substance. Traditionally, the simplest method has consisted in placing a receptacle on a weighing member, the receptacle itself being located beneath a filler member which is switched on and off under the control of the weighing member as a function of the apparent weight measured thereby. The apparent weight includes not only the weight of the receptacle and the net weight of substance contained in the receptacle, but also the force that results from the jet of substance striking the surface of substance already contained in the receptacle. This force varies not only as a function of the degree of opening of the filler member, but also as a function of the viscosity of the substance, which means that if the viscosity of the substance varies during filling, then the weight measurement performed by the weighing member is erroneous and the real net weight of substance filled into the receptacle at the end of weighing is not equal to the desired net weight of substance.
In addition, at the moment the filler member is switched off, the weight of substance extending between the filler member and the surface of the substance already in the receptacle, referred to as herein as the "tail-back" weight, is added to the weight of substance finally contained in the receptacle at the end of the filling cycle. The weight of the tail-back varies as a function of the diameter of the filler member orifice immediately prior to being turned off, and on the viscosity of the substance. In conventional methods, the pressure of the jet during filling and the weight of the tail-back must therefore be compensated so that in the end the desired reference net weight of substance is obtained in the receptacle.
French patent 2 679 516 discloses a filling method consisting in servo-controlling the flow rate of the substance to a reference flow rate, and in performing filling for a predetermined fixed length of time that is calculated in advance by dividing the reference net weight by the reference flow rate. That method makes it possible to eliminate the effects of the pressure of the jet on the surface of substance already contained in the receptacle by measuring the instantaneous flow rate over successive time intervals during which the force of the jet of substance on the surface of the substance already contained in the receptacle is assumed to remain constant. In theory, that method ought to be highly satisfactory, however, in practice, servo-controlling the flow rate to a reference flow rate does not make it possible to obtain a real flow rate that is exactly equal to the reference flow rate, and it is therefore necessary to perform compensation by checking the real net weight after filling and by modifying the parameters of the servo-control loop for subsequent filling cycles so that the real net weight becomes as close as possible to the reference net weight.